IL NUOVO CIMENTO Vol. ?, N. ? ? Multiwavelength chase of GRB 031220 afterglow A. Melandri( 1 )( 2 ), B. Gendre( 3 ), L. A. Antonelli( 1 ), A. Grazian( 1 ), A. de Ugarte( 4 ), J. Gorosabel( 4 ), L. Piro( 3 ), G. Kosugi( 5 ), N. Kaway( 6 ) ( 1 ) INAF - Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone, Italy ( 2 ) Universit´ a di Cagliari, Piazza d’Armi, 09123 Cagliari, Italy ( 3 ) Istituto di Astrofisica Spaziale e Fisica Cosmica-CNR, Via Fosso del Cavaliere 100, 00133 Roma, Italy ( 4 ) Istituto de Astrofisica de Andalucia (IAA-CSIC), Camino Bajo de Huetor 24, 18.008 Granada, Spain ( 5 ) Department of Physics, Tokyo Institute of Technology, Meguro, Tokyo 152-0033 ( 6 ) Subaru Telescope, National Astronomical Observatory of Japan, Hilo, HI 96720, USA Summary. — Several gamma ray bursts (GRBs) with X-ray afterglow do not show any optical-IR afterglow. The nature of this class of events, the so called Dark Bursts, is still not clear. The optical absorption could be due to the interstellar dust or to the high redshift of the event. Or, more simply, the non-detection of the optical transient should be due to the delay in the observation or to the rapid energy decaying of these events. High spatial resolution X-ray observations are the most promising tool to investigate on such kind of events. We have collected and analized X-ray data and images taken in different spectral bands (optical and infrared) for GRB 031220 and we present the results of the analysis of multiband observations on the field of this burst. Comparison between images taken at different epochs in the same filters did not reveal any strongly variable sources. Photometric analysis and photometric redshift estimation of all possible afterglow candidates suggest that this GRB can be classified as a Dark Burst. 1. – Introdution The lack of optical afterglow for a large fraction (about 60-70%) of well localized X-ray afterglows and, sometimes, together with the detection of radio afterglow, leads to the definition of the phenomenological class of the so called Dark Bursts. The nature of this class of events, bursts with no apparent optical afterglow, is still not clear but recent works have suggested some possible scenarios. In the obscuration scenario the failed detection of the optical afterglow is ascribed to extinction by the dust of the host galaxy. Instead, in the high-redshift scenario, the afterglow emission is dumped by the c ⃝ Societ` a Italiana di Fisica 1